solventaccessible

Solventaccessible, in the context of biomolecules, refers to the degree to which a molecule or part of a molecule is exposed to solvent, typically water. In protein biochemistry, this concept is formalized through solvent-accessible surface area (SASA), which measures how much surface area of atoms is accessible to a solvent probe.

SASA is computed by modeling the molecule and rolling a spherical probe, usually with a radius of about 1.4 Å (the approximate size of a water molecule), over the van der Waals surface. The surface traced by the center of the probe constitutes the solvent-accessible surface. Various algorithms implement this idea, with common approaches developed by Lee and Richards and by Shrake and Rupley. The resulting SASA values depend on molecular conformation, atom radii, and probe size, and can be calculated for individual atoms, residues, or entire chains.

To compare exposure across different residue types, SASA is often normalized to yield relative solvent accessibility (RSA). RSA is the observed SASA divided by the maximum SASA possible for that residue type, producing a value from 0 (completely buried) to 1 (fully exposed). In practice, thresholds vary, but RSA provides a convenient way to classify residues as buried or exposed.

Solvent accessibility is important for understanding protein stability, folding, and interactions. Buried residues typically contribute to the hydrophobic core and stability, while exposed residues participate in solubility, binding interfaces, and catalysis. Changes in solvent accessibility during conformational shifts can indicate functional movements, allosteric regulation, or hole formation in active sites.

Several software tools implement SASA and RSA calculations, including widely used packages in structural biology. Limitations include sensitivity to the chosen conformation, probe size, and radii conventions, as well as the dynamic nature of solvent interactions not captured in a single static model.